In recent years, metal oxide surge arresters for protecting outdoor overhead high voltage electrical systems against lightning have been advantageously provided with polymeric weathershed housings, disconnectors, and with insulative brackets. Metal oxide varistor blocks are sealed within the polymeric housings, which protect and insulate the varistor blocks against the damaging effects of rain, snow and airborne contamination, for example. The insulative brackets have provided electrical insulation to allow for proper operation of the ground lead disconnector.
Prior art surge arresters have also included fiberglass tubes or wrappings supporting the varistor blocks. Also, the inner diameter, or bore, of the polymeric housings have been bonded to the outer peripheral surfaces of the tube or wrapping. U.S. Pat. Nos. 4,656,555 and 4,404,614 disclose some embodiments of these constructions and methods of construction. Methods of bonding a polymeric housing along its bore directly to varistor blocks, or to layers or coatings surrounding the blocks, are also disclosed in U.S. Pat. No. 4,161,012. Polymer housings molded directly onto the peripheral surfaces of an arrester element as well as to flat end surfaces are disclosed in U.S. Pat. No. 4,833,438. Each of the methods and embodiments referred to in the afore-mentioned U.S. patents involves a relatively expensive process for achieving a seal between the inner surfaces of an arrester housing and the outer or peripheral surfaces of arrester components enclosed within the housing.
A method of achieving an air free, sealed, moisture excluding interface between peripheral surfaces of arrester components and inner surfaces of an elastomeric housing that includes the use of silicone grease at the interface is also described with reference to FIG. 8 in U.S. Pat. No. 4,161,012. In a prior art embodiment using silicone grease at the above-described interface, the elastomeric or polymeric housing surrounding the arrester components has been also compressed between flat surfaces disposed at the opposing flat ends of the housing. However, in this embodiment, silicone oil from the silicone grease has escaped, along the flat surfaces, to outer surfaces of the housing. Here, the oil has accumulated objectionable quantities of airborne dust and dirt.
A surge arrester insulative bracket is shown in FIG. 3 of U.S. Pat. No. 4,609,902. Others are identified as the insulating base shown on pages 10 and 11 of Ohio Brass Catalog 94. The method of bolting the insulating base to a metallic arrester mounting bracket is shown in detail on Ohio Brass Instruction Sheet No. 17-5113, whereon the recommended mounting torque for the one-half inch fastener is forty ft-lbs. Torque greater than forty ft-lbs has cracked the insulating base in the area of the bolt hole extending through the insulating base. Restricting the applied torque to forty ft-lbs places the burden of not cracking the insulating base, i.e., the insulative bracket, upon the skill, knowledge and diligence of the installer.
In the prior art devices, a lockwasher on a mounting bolt of the metallic bracket is compressed against a flat surface of an insulative bracket, the flat surface being perpendicular to the bolt hole extending through the bracket. The tightening of a threaded nut against the lockwasher transmits a mechanical force through the lockwasher to the flat surface. At a level of mechanical force predetermined by the characteristics of the insulative mass of material forming the insulative bracket, the flat surface yields and the insulative material under the lockwasher is crushed into the clearance that had surrounded the bolt in the bolt hole. Further tightening of the nut against the lockwasher then produces radial cracks extending from the damaged bolt hole through the insulative material, thereby weakening the bracket and drawing complaints from users.
Thus there is a need for an improved insulating base that can withstand greater torque during installation.
After a surge arrester has been bolted to a metallic mounting bracket, a threaded nut must be tightened against a clamp at the top end of the arrester to connect the arrester to an electrical power system. Similarly, a threaded nut at the bottom end of the arrester must be tightened to connect the ground lead disconnector of the arrester to a source of ground potential. During the tightening of either of these threaded nuts, there has been a tendency for the arrester housing, including the varistor blocks within the housing, to rotate with respect to the arrester insulative bracket, thereby drawing complaints from users. Thus, there is a need to prevent such rotation.
In use, the metallic mounting bracket may also be connected to a source of ground potential.
With all connections made as described above, the full voltage of the power system exists across the arrester, from the top end of the arrester to the bottom end; and little voltage, if any, is impressed across the insulative bracket. However, the internal components of surge arresters have been known to become damaged, in which case the disconnector will automatically disconnect the ground wire from the arrester. Now the power line voltage may be impressed across the insulative bracket, as is known to those skilled in the art, until such time as the damaged arrester is discovered and replaced. Damaged arresters may not be discovered and replaced for hours, days, or even months, during which time the insulative bracket must withstand the voltage across it, without regard for weather conditions or atmosphere borne contaminants that may become deposited on the insulative bracket.
Under damp or rain conditions, the contaminants and moisture combine on the broad upper surfaces of prior art insulative brackets and conduct electrical leakage current across those upper surfaces as well as up and down the vertical surfaces of interposed baffles formed on the insulative brackets. Eventually, in the course of wetting and drying, the electrical current flowing across those surfaces, including the baffle surfaces, may damage the insulative bracket and/or cause it to fail to withstand the impressed voltage, thereby causing an outage of the electrical system. Baffles are known to be raised portions of insulative material disposed transversely to a leakage current path along an insulative bracket as a means to increase the length of the creepage distance, and thereby to decrease the magnitude of the leakage current.